In epistemology, the Münchhausen trilemma is a thought experiment used to demonstrate the impossibility of proving
any truth, even in the fields of logic and mathematics. If it is asked
how any given proposition is known to be true, proof may be provided.
Yet that same question can be asked of the proof, and any subsequent
proof. The Münchhausen trilemma is that there are only three ways of
completing a proof:
The circular argument, in which the proof of some proposition is supported only by that proposition
The dogmatic argument, which rests on accepted precepts which are merely asserted rather than defended
The trilemma, then, is the decision among the three equally unsatisfying options.
The name Münchhausen-Trilemma was coined by the German philosopher Hans Albert in 1968 in reference to a trilemma of "dogmatism versus infinite regress versus psychologism" used by Karl Popper. It is a reference to the problem of "bootstrapping", based on the story of Baron Munchausen (in German, "Münchhausen") pulling himself and the horse on which he was sitting out of a mire
by his own hair. Like Munchausen, who cannot make progress because he
has no solid ground to stand on, any purported justification of all
knowledge must fail, because it must start from a position of no
knowledge, and therefore cannot make progress. It must either start with
some knowledge, as with dogmatism, not start at all, as with infinite regress, or be a circular argument,
justified only by itself and have no solid foundation, much like
Munchausen is only pulling on himself rather than some external
handhold.
It is also known as Agrippa's trilemma or the Agrippan trilemma after a similar argument reported by Sextus Empiricus, which was attributed to Agrippa the Skeptic by Diogenes Laërtius.
Sextus' argument, however, consists of five (not three) "modes". Popper
in his original 1935 publication mentions neither Sextus nor Agrippa,
but instead attributes his trilemma to German philosopher Jakob Friedrich Fries, leading some to call it Fries's trilemma as a result.
Relation – All things are changed as their relations become changed, or, as they are looked upon from different points of view.
Assumption – The truth asserted is based on an unsupported assumption.
Circularity – the truth asserted involves a circularity of proofs (known in scholasticism as "diallelus")
According to the mode deriving from dispute, we find that undecidable dissension
about the matter proposed has come about both in ordinary life and
among philosophers. Because of this we are not able to choose or to
rule out anything, and we end up with suspension of judgement.
In the mode deriving from infinite regress, we say that what is brought
forward as a source of conviction for the matter proposed itself needs
another such source, which itself needs another, and so ad infinitum,
so that we have no point from which to begin to establish anything, and
suspension of judgement follows. In the mode deriving from relativity,
as we said above, the existing object appears to be such-and-such
relative to the subject judging and to the things observed together with
it, but we suspend judgement on what it is like in its nature. We have
the mode from hypothesis when the Dogmatists, being thrown back ad infinitum,
begin from something which they do not establish but claim to assume
simply and without proof in virtue of a concession. The reciprocal mode
occurs when what ought to be confirmatory of the object under
investigation needs to be made convincing by the object under
investigation; then, being unable to take either in order to establish
the other, we suspend judgement about both.
With reference to these five tropes, the first and third are a short summary of the ten modes of Aenesidemus which were the basis of earlier Pyrrhonism.
The three additional ones show a progress in the Pyrrhonist system, and
a transition from the common objections derived from the fallibility of
sense and opinion, to more abstract and metaphysical grounds of
skepticism.
According to Victor Brochard,
"the five tropes can be regarded as the most radical and most precise
formulation of skepticism that has ever been given. In a sense, they
are still irresistible today."
Fries's trilemma
Jakob Friedrich Fries formulated a similar trilemma in which statements can be accepted either:
The first two possibilities are rejected by Fries as unsatisfactory, requiring his adopting the third option. Karl Popper
argued that a way to avoid the trilemma was to use an intermediate
approach incorporating some dogmatism, some infinite regress, and some
perceptual experience.
Albert's formulation
The argument proposed by Hans Albert runs as follows: All of the only three possible attempts to get a certain justification must fail:
All justifications in pursuit of "certain" knowledge have also
to justify the means of their justification and doing so they have to
justify anew the means of their justification. Therefore, there can be
no end, only the hopeless situation of infinite regression
A circular argument can be used to justify by its mock impression of
validity and soundness, but this sacrifices its usefulness (as the
conclusion and premise are one and the same, no advancement in knowledge
has taken place).
One can stop at self-evidence or common sense or fundamental principles or speaking ex cathedra or at any other evidence, but in doing so, the intention to install 'certain' justification is abandoned.
An English translation of a quote from the original German text by Albert is as follows:
Here, one has a mere choice between:
An infinite regression, which appears because of the
necessity to go ever further back, but is not practically feasible and
does not, therefore, provide a certain foundation.
A logical circle in the deduction, which is caused by the fact that
one, in the need to found, falls back on statements which had already
appeared before as requiring a foundation, and which circle does not
lead to any certain foundation either.
A break of searching at a certain point, which indeed appears principally feasible, but would mean a random suspension of the principle of sufficient reason.
Albert stressed repeatedly that there is no limitation of the
Münchhausen trilemma to deductive conclusions. The verdict concerns also
inductive, causal, transcendental, and all otherwise structured
justifications. They all will be in vain.
Therefore, certain justification is impossible to attain. Once
having given up the classical idea of certain knowledge, one can stop
the process of justification where one wants to stop, presupposed one is
ready to start critical thinking at this point always anew if
necessary.
The failure of proving exactly any truth as expressed by the
Münchhausen trilemma does not have to lead to dismissal of objectivity,
as with relativism. One example of an alternative is the fallibilism of Karl Popper and Hans Albert, accepting that certainty is impossible, but that it is best to get as close as possible to truth, while remembering our uncertainty.
In Albert's view, the impossibility to prove any certain truth is
not in itself a certain truth. After all, one needs to assume some
basic rules of logical inference to derive his result, and in doing so
must either abandon the pursuit of "certain" justification, as above, or
attempt to justify these rules, etc. He suggests that it has to be
taken as true as long as nobody has come forward with a truth which is
scrupulously justified as a certain truth. Several philosophers defied
Albert's challenge; his responses to such criticisms can be found in his
long addendum to his Treatise on Critical Reason and later articles.
The human circulatory system (simplified). Red indicates oxygenated blood carried in arteries. Blue indicates deoxygenated blood carried in veins. Capillaries, which join the arteries and veins, and the lymphatic vessels are not shown.
The circulatory system includes the lymphatic system, which circulates lymph. The passage of lymph takes much longer than that of blood. Blood is a fluid consisting of plasma, red blood cells, white blood cells, and platelets that is circulated by the heart
through the vertebrate vascular system, carrying oxygen and nutrients
to and waste materials away from all body tissues. Lymph is essentially
recycled excess blood plasma after it has been filtered from the interstitial fluid
(between cells) and returned to the lymphatic system. The
cardiovascular (from Latin words meaning "heart" and "vessel") system
comprises the blood, heart, and blood vessels. The lymph, lymph nodes, and lymph vessels form the lymphatic system, which returns filtered blood plasma from the interstitial fluid (between cells) as lymph.
The circulatory system of the blood has two components, a systemic circulation and a pulmonary circulation. While humans and other vertebrates have a closed cardiovascular system (which means that the blood never leaves the network of arteries, veins and capillaries), some invertebrate
groups have an open cardiovascular system. The lymphatic system, in
contrast, is an open system providing an accessory route for excess
interstitial fluid to be returned to the blood. The more primitive, diploblastic animal phyla lack circulatory systems.
Many diseases affect the circulatory system. This includes cardiovascular disease, affecting the cardiovascular system, and lymphatic disease affecting the lymphatic system. Cardiologists are medical professionals which specialise in the heart, and cardiothoracic surgeons specialise in operating on the heart and its surrounding areas. Vascular surgeons focus on other parts of the circulatory system.
Structure
Cardiovascular system
Depiction of the heart, major veins and arteries constructed from body scans
Cross section of a human artery
The essential components of the human cardiovascular system are the heart, blood and blood vessels. It includes the pulmonary circulation, a "loop" through the lungs where blood is oxygenated; and the systemic circulation, a "loop" through the rest of the body to provide oxygenated blood. The systemic circulation can also be seen to function in two parts – a macrocirculation and a microcirculation.
An average adult contains five to six quarts (roughly 4.7 to 5.7
liters) of blood, accounting for approximately 7% of their total body
weight. Blood consists of plasma, red blood cells, white blood cells, and platelets. Also, the digestive system works with the circulatory system to provide the nutrients the system needs to keep the heart pumping.
The cardiovascular systems of humans are closed, meaning that the blood never leaves the network of blood vessels. In contrast, oxygen and nutrients diffuse across the blood vessel layers and enter interstitial fluid,
which carries oxygen and nutrients to the target cells, and carbon
dioxide and wastes in the opposite direction. The other component of the
circulatory system, the lymphatic system, is open.
Arteries
Oxygenated blood enters the systemic circulation when leaving the left ventricle, through the aortic semilunar valve. The first part of the systemic circulation is the aorta,
a massive and thick-walled artery. The aorta arches and gives branches
supplying the upper part of the body after passing through the aortic
opening of the diaphragm at the level of thoracic ten vertebra, it
enters the abdomen. Later it descends down and supplies branches to
abdomen, pelvis, perineum and the lower limbs. The walls of aorta are
elastic. This elasticity helps to maintain the blood pressure
throughout the body. When the aorta receives almost five litres of
blood from the heart, it recoils and is responsible for pulsating blood
pressure. Moreover, as aorta branches into smaller arteries, their
elasticity goes on decreasing and their compliance goes on increasing.
Capillaries
Arteries branch into small passages called arterioles and then into the capillaries. The capillaries merge to bring blood into the venous system.
Veins
Capillaries merge into venules, which merge into veins. The venous system feeds into the two major veins: the superior vena cava – which mainly drains tissues above the heart – and the inferior vena cava – which mainly drains tissues below the heart. These two large veins empty into the right atrium of the heart.
Portal veins
The general rule is that arteries from the heart branch out into
capillaries, which collect into veins leading back to the heart. Portal veins are a slight exception to this. In humans the only significant example is the hepatic portal vein which combines from capillaries around the gastrointestinal tract
where the blood absorbs the various products of digestion; rather than
leading directly back to the heart, the hepatic portal vein branches
into a second capillary system in the liver.
Heart
View from the front
The heart pumps oxygenated blood to the body and deoxygenated blood to the lungs. In the human heart there is one atrium and one ventricle for each circulation, and with both a systemic and a pulmonary circulation there are four chambers in total: left atrium, left ventricle, right atrium and right ventricle.
The right atrium is the upper chamber of the right side of the heart.
The blood that is returned to the right atrium is deoxygenated (poor in
oxygen) and passed into the right ventricle to be pumped through the
pulmonary artery to the lungs for re-oxygenation and removal of carbon
dioxide. The left atrium receives newly oxygenated blood from the lungs
as well as the pulmonary vein which is passed into the strong left
ventricle to be pumped through the aorta to the different organs of the
body.
Coronary vessels
The heart itself is supplied with oxygen and nutrients through a
small "loop" of the systemic circulation and derives very little from
the blood contained within the four chambers.
The coronary circulation system provides a blood supply to the heart muscle itself. The coronary circulation begins near the origin of the aorta by two coronary arteries: the right coronary artery and the left coronary artery. After nourishing the heart muscle, blood returns through the coronary veins into the coronary sinus and from this one into the right atrium. Back flow of blood through its opening during atrial systole is prevented by Thebesian valve. The smallest cardiac veins drain directly into the heart chambers.
Oxygen-deprived blood from the superior and inferior vena cava enters the right atrium of the heart and flows through the tricuspid valve (right atrioventricular valve) into the right ventricle, from which it is then pumped through the pulmonary semilunar valve into the pulmonary artery to the lungs. Gas exchange occurs in the lungs, whereby CO 2 is released from the blood, and oxygen is absorbed. The pulmonary vein returns the now oxygen-rich blood to the left atrium.
A separate system known as the bronchial circulation supplies blood to the tissue of the larger airways of the lung.
Systemic circulation
The systemic circulation and capillary networks shown and also as separate from the pulmonary circulation
Systemic circulation is the portion of the cardiovascular system
which transports oxygenated blood away from the heart through the aorta
from the left ventricle where the blood has been previously deposited
from pulmonary circulation, to the rest of the body, and returns
oxygen-depleted blood back to the heart.
Brain
The brain has a dual blood supply that comes from arteries at its
front and back. These are called the "anterior" and "posterior"
circulation respectively. The anterior circulation arises from the internal carotid arteries and supplies the front of the brain. The posterior circulation arises from the vertebral arteries, and supplies the back of the brain and brainstem. The circulation from the front and the back join together (anastomise) at the Circle of Willis.
Kidneys
The renal circulation receives around 20% of the cardiac output. It branches from the abdominal aorta and returns blood to the ascending vena cava. It is the blood supply to the kidneys, and contains many specialized blood vessels.
The development of the circulatory system starts with vasculogenesis in the embryo.
The human arterial and venous systems develop from different areas in
the embryo. The arterial system develops mainly from the aortic arches,
six pairs of arches that develop on the upper part of the embryo. The
venous system arises from three bilateral veins during weeks 4 – 8 of embryogenesis. Fetal circulation begins within the 8th week of development. Fetal circulation does not include the lungs, which are bypassed via the truncus arteriosus. Before birth the fetus obtains oxygen (and nutrients) from the mother through the placenta and the umbilical cord.
Heart
Arteries
Animation
of a typical human red blood cell cycle in the circulatory system. This
animation occurs at a faster rate (~20 seconds of the average 60-second cycle)
and shows the red blood cell deforming as it enters capillaries, as
well as the bars changing color as the cell alternates in states of
oxygenation along the circulatory system.
The human arterial system originates from the aortic arches and from the dorsal aortae starting from week 4 of embryonic life. The first and second aortic arches regress and form only the maxillary arteries and stapedial arteries respectively. The arterial system itself arises from aortic arches 3, 4 and 6 (aortic arch 5 completely regresses).
The dorsal aortae, present on the dorsal side of the embryo, are initially present on both sides of the embryo. They later fuse to form the basis for the aorta itself. Approximately thirty smaller arteries branch from this at the back and sides. These branches form the intercostal arteries,
arteries of the arms and legs, lumbar arteries and the lateral sacral
arteries. Branches to the sides of the aorta will form the definitive renal, suprarenal and gonadal arteries. Finally, branches at the front of the aorta consist of the vitelline arteries and umbilical arteries. The vitelline arteries form the celiac, superior and inferior mesenteric arteries of the gastrointestinal tract. After birth, the umbilical arteries will form the internal iliac arteries.
About 98.5% of the oxygen in a sample of arterial blood in a healthy human, breathing air at sea-level pressure, is chemically combined with hemoglobin
molecules. About 1.5% is physically dissolved in the other blood
liquids and not connected to hemoglobin. The hemoglobin molecule is the
primary transporter of oxygen in mammals and many other species.
Lymphatic system
Clinical significance
Many diseases affect the circulatory system. These include a number of cardiovascular diseases, affecting the cardiovascular system, and lymphatic diseases affecting the lymphatic system. Cardiologists are medical professionals which specialise in the heart, and cardiothoracic surgeons specialise in operating on the heart and its surrounding areas. Vascular surgeons focus on other parts of the circulatory system.
Many of these diseases are called "lifestyle diseases"
because they develop over time and are related to a person's exercise
habits, diet, whether they smoke, and other lifestyle choices a person
makes. Atherosclerosis is the precursor to many of these diseases. It is where small atheromatous plaques
build up in the walls of medium and large arteries. This may eventually
grow or rupture to occlude the arteries. It is also a risk factor for acute coronary syndromes,
which are diseases that are characterised by a sudden deficit of
oxygenated blood to the heart tissue. Atherosclerosis is also associated
with problems such as aneurysm formation or splitting ("dissection") of arteries.
Another major cardiovascular disease involves the creation of a clot, called a "thrombus". These can originate in veins or arteries. Deep venous thrombosis,
which mostly occurs in the legs, is one cause of clots in the veins of
the legs, particularly when a person has been stationary for a long
time. These clots may embolise, meaning travel to another location in the body. The results of this may include pulmonary embolus, transient ischaemic attacks, or stroke.
Cardiovascular diseases may also be congenital in nature, such as heart defects or persistent fetal circulation,
where the circulatory changes that are supposed to happen after birth
do not. Not all congenital changes to the circulatory system are
associated with diseases, a large number are anatomical variations.
The function and health of the circulatory system and its parts are
measured in a variety of manual and automated ways. These include simple
methods such as those that are part of the cardiovascular examination, including the taking of a person's pulse as an indicator of a person's heart rate, the taking of blood pressure through a sphygmomanometer or the use of a stethoscope to listen to the heart for murmurs which may indicate problems with the heart's valves. An electrocardiogram can also be used to evaluate the way in which electricity is conducted through the heart.
Other more invasive means can also be used. A cannula or catheter inserted into an artery may be used to measure pulse pressure or pulmonary wedge pressures. Angiography, which involves injecting a dye into an artery to visualise an arterial tree, can be used in the heart (coronary angiography) or brain. At the same time as the arteries are visualised, blockages or narrowings may be fixed through the insertion of stents, and active bleeds may be managed by the insertion of coils. An MRI may be used to image arteries, called an MRI angiogram. For evaluation of the blood supply to the lungs a CT pulmonary angiogram may be used.
Cardiovascular procedures are more likely to be performed in an
inpatient setting than in an ambulatory care setting; in the United
States, only 28% of cardiovascular surgeries were performed in the
ambulatory care setting.[13]
Society and culture
In Ancient Greece, the heart was thought to be the source of innate heat for the body.
The circulatory system as we know it was discovered by William Harvey.
Other animals
The
open circulatory system of the grasshopper – made up of a heart,
vessels and hemolymph. The hemolymph is pumped through the heart, into
the aorta, dispersed into the head and throughout the hemocoel, then
back through the ostia in the heart and the process repeated.
While humans, as well as other vertebrates, have a closed blood circulatory system (meaning that the blood never leaves the network of arteries, veins and capillaries), some invertebrate groups have an open circulatory system containing a heart but limited blood vessels. The most primitive, diploblastic animal phyla lack circulatory systems.
An additional transport system, the lymphatic system, which is
only found in animals with a closed blood circulation, is an open
system providing an accessory route for excess interstitial fluid to be
returned to the blood.
The blood vascular system first appeared probably in an ancestor of the triploblasts over 600 million years ago, overcoming the time-distance constraints of diffusion, while endothelium evolved in an ancestral vertebrate some 540–510 million years ago.
In arthropods,
the open circulatory system is a system in which a fluid in a cavity
called the hemocoel bathes the organs directly with oxygen and
nutrients, with there being no distinction between blood and interstitial fluid; this combined fluid is called hemolymph or haemolymph. Muscular movements by the animal during locomotion can facilitate hemolymph movement, but diverting flow from one area to another is limited. When the heart relaxes, blood is drawn back toward the heart through open-ended pores (ostia).
There are free-floating cells, the hemocytes, within the hemolymph. They play a role in the arthropod immune system.
Flatworms, such as this Pseudoceros bifurcus, lack specialized circulatory organs.
Closed circulatory system
Two-chambered heart of a fish
The circulatory systems of all vertebrates, as well as of annelids (for example, earthworms) and cephalopods (squids, octopuses and relatives) always keep their circulating blood enclosed within heart chambers or blood vessels and are classified as closed, just as in humans. Still, the systems of fish, amphibians, reptiles, and birds show various stages of the evolution of the circulatory system. Closed systems permit blood to be directed to the organs that require it.
In fish, the system has only one circuit, with the blood being pumped through the capillaries of the gills and on to the capillaries of the body tissues. This is known as single cycle circulation. The heart of fish is, therefore, only a single pump (consisting of two chambers).
In amphibians and most reptiles, a double circulatory system is
used, but the heart is not always completely separated into two pumps.
Amphibians have a three-chambered heart.
In reptiles, the ventricular septum of the heart is incomplete and the pulmonary artery is equipped with a sphincter muscle.
This allows a second possible route of blood flow. Instead of blood
flowing through the pulmonary artery to the lungs, the sphincter may be
contracted to divert this blood flow through the incomplete ventricular
septum into the left ventricle and out through the aorta.
This means the blood flows from the capillaries to the heart and back
to the capillaries instead of to the lungs. This process is useful to ectothermic (cold-blooded) animals in the regulation of their body temperature.
Birds, mammals, and crocodilians
show complete separation of the heart into two pumps, for a total of
four heart chambers; it is thought that the four-chambered heart of
birds and crocodilians evolved independently from that of mammals.
Double circulatory systems permit blood to be repressurized after
returning from the lungs, speeding up delivery of oxygen to tissues.
No circulatory system
Circulatory systems are absent in some animals, including flatworms. Their body cavity has no lining or enclosed fluid. Instead, a muscular pharynx leads to an extensively branched digestive system that facilitates direct diffusion
of nutrients to all cells. The flatworm's dorso-ventrally flattened
body shape also restricts the distance of any cell from the digestive
system or the exterior of the organism. Oxygen can diffuse from the surrounding water into the cells, and carbon dioxide can diffuse out. Consequently, every cell is able to obtain nutrients, water and oxygen without the need of a transport system.
Some animals, such as jellyfish, have more extensive branching from their gastrovascular cavity
(which functions as both a place of digestion and a form of
circulation), this branching allows for bodily fluids to reach the outer
layers, since the digestion begins in the inner layers.
History
Human
anatomical chart of blood vessels, with heart, lungs, liver and kidneys
included. Other organs are numbered and arranged around it. Before
cutting out the figures on this page, Vesalius
suggests that readers glue the page onto parchment and gives
instructions on how to assemble the pieces and paste the multilayered
figure onto a base "muscle man" illustration. "Epitome", fol.14a. HMD
Collection, WZ 240 V575dhZ 1543.
The earliest known writings on the circulatory system are found in the Ebers Papyrus (16th century BCE), an ancient Egyptian medical papyrus containing over 700 prescriptions and remedies, both physical and spiritual. In the papyrus,
it acknowledges the connection of the heart to the arteries. The
Egyptians thought air came in through the mouth and into the lungs and
heart. From the heart, the air travelled to every member through the
arteries. Although this concept of the circulatory system is only
partially correct, it represents one of the earliest accounts of
scientific thought.
In the 6th century BCE, the knowledge of circulation of vital fluids through the body was known to the Ayurvedic physician Sushruta in ancient India. He also seems to have possessed knowledge of the arteries, described as 'channels' by Dwivedi & Dwivedi (2007). The valves of the heart were discovered by a physician of the Hippocratean
school around the 4th century BCE. However, their function was not
properly understood then. Because blood pools in the veins after death,
arteries look empty. Ancient anatomists assumed they were filled with
air and that they were for the transport of air.
The Greek physician, Herophilus, distinguished veins from arteries but thought that the pulse was a property of arteries themselves. Greek anatomist Erasistratus
observed that arteries that were cut during life bleed. He ascribed the
fact to the phenomenon that air escaping from an artery is replaced
with blood that entered by very small vessels between veins and
arteries. Thus he apparently postulated capillaries but with reversed
flow of blood.
In 2nd-century AD Rome, the Greek physician Galen
knew that blood vessels carried blood and identified venous (dark red)
and arterial (brighter and thinner) blood, each with distinct and
separate functions. Growth and energy were derived from venous blood
created in the liver from chyle, while arterial blood gave vitality by
containing pneuma (air) and originated in the heart. Blood flowed from
both creating organs to all parts of the body where it was consumed and
there was no return of blood to the heart or liver. The heart did not
pump blood around, the heart's motion sucked blood in during diastole
and the blood moved by the pulsation of the arteries themselves.
Galen believed that the arterial blood was created by venous
blood passing from the left ventricle to the right by passing through
'pores' in the interventricular septum, air passed from the lungs via
the pulmonary artery to the left side of the heart. As the arterial
blood was created 'sooty' vapors were created and passed to the lungs
also via the pulmonary artery to be exhaled.
In 1025, The Canon of Medicine by the Persian physician, Avicenna,
"erroneously accepted the Greek notion regarding the existence of a
hole in the ventricular septum by which the blood traveled between the
ventricles." Despite this, Avicenna "correctly wrote on the cardiac cycles and valvular function", and "had a vision of blood circulation" in his Treatise on Pulse.
While also refining Galen's erroneous theory of the pulse, Avicenna
provided the first correct explanation of pulsation: "Every beat of the
pulse comprises two movements and two pauses. Thus, expansion : pause :
contraction : pause. [...] The pulse is a movement in the heart and
arteries ... which takes the form of alternate expansion and
contraction."
"...the blood from the right chamber of the heart must
arrive at the left chamber but there is no direct pathway between them.
The thick septum of the heart is not perforated and does not have
visible pores as some people thought or invisible pores as Galen
thought. The blood from the right chamber must flow through the vena
arteriosa (pulmonary artery) to the lungs, spread through its substances, be mingled there with air, pass through the arteria venosa (pulmonary vein) to reach the left chamber of the heart and there form the vital spirit..."
In addition, Ibn al-Nafis had an insight into what would become a larger theory of the capillary circulation. He stated that "there must be small communications or pores (manafidh
in Arabic) between the pulmonary artery and vein," a prediction that
preceded the discovery of the capillary system by more than 400 years. Ibn al-Nafis' theory, however, was confined to blood transit in the lungs and did not extend to the entire body.
Michael Servetus
was the first European to describe the function of pulmonary
circulation, although his achievement was not widely recognized at the
time, for a few reasons. He firstly described it in the "Manuscript of
Paris" (near 1546), but this work was never published. And later he published this description, but in a theological treatise, Christianismi Restitutio,
not in a book on medicine. Only three copies of the book survived but
these remained hidden for decades, the rest were burned shortly after
its publication in 1553 because of persecution of Servetus by religious
authorities.
Better known discovery of pulmonary circulation was by Vesalius's successor at Padua, Realdo Colombo, in 1559.
Finally, the English physician William Harvey, a pupil of Hieronymus Fabricius
(who had earlier described the valves of the veins without recognizing
their function), performed a sequence of experiments and published his Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus
in 1628, which "demonstrated that there had to be a direct connection
between the venous and arterial systems throughout the body, and not
just the lungs. Most importantly, he argued that the beat of the heart
produced a continuous circulation of blood through minute connections at
the extremities of the body. This is a conceptual leap that was quite
different from Ibn al-Nafis' refinement of the anatomy and bloodflow in
the heart and lungs."
This work, with its essentially correct exposition, slowly convinced
the medical world. However, Harvey was not able to identify the
capillary system connecting arteries and veins; these were later
discovered by Marcello Malpighi in 1661.
All
cardiologists study the disorders of the heart, but the study of adult
and child heart disorders are through different training pathways.
Therefore, an adult cardiologist (often simply called "cardiologist") is
inadequately trained to take care of children, and pediatric
cardiologists are not trained to take care of adult heart disease. The
surgical aspects are not included in cardiology and are in the domain of
cardiothoracic surgery. For example, coronary artery bypass surgery (CABG), cardiopulmonary bypass and valve replacement
are surgical procedures performed by surgeons, not cardiologists.
However, the insertion of stents and pacemakers is performed by
cardiologists.
Per Doximity, adult cardiologists make an average of $436,849 in the United States.
Cardiac electrophysiology
Cardiac electrophysiology is the science of elucidating, diagnosing, and treating the electrical activities of the heart.
The term is usually used to describe studies of such phenomena by
invasive (intracardiac) catheter recording of spontaneous activity as
well as of cardiac responses to programmed electrical stimulation (PES). These studies are performed to assess complex arrhythmias, elucidate symptoms, evaluate abnormal electrocardiograms,
assess risk of developing arrhythmias in the future, and design
treatment. These procedures increasingly include therapeutic methods
(typically radiofrequency ablation, or cryoablation) in addition to diagnostic and prognostic procedures. Other therapeutic modalities employed in this field include antiarrhythmic drug therapy and implantation of pacemakers and automatic implantable cardioverter-defibrillators (AICD).
The cardiac electrophysiology study
(EPS) typically measures the response of the injured or cardiomyopathic
myocardium to PES on specific pharmacological regimens in order to
assess the likelihood that the regimen will successfully prevent
potentially fatal sustained ventricular tachycardia (VT) or ventricular fibrillation (VF) in the future. Sometimes a series
of EPS drug trials must be conducted to enable the cardiologist to
select the one regimen for long-term treatment that best prevents or
slows the development of VT or VF following PES. Such studies may also
be conducted in the presence of a newly implanted or newly replaced
cardiac pacemaker or AICD.
Clinical cardiac electrophysiology
Clinical cardiac electrophysiology
is a branch of the medical specialty of cardiology and is concerned
with the study and treatment of rhythm disorders of the heart.
Cardiologists with expertise in this area are usually referred to as
electrophysiologists. Electrophysiologists are trained in the mechanism,
function, and performance of the electrical activities of the heart.
Electrophysiologists work closely with other cardiologists and cardiac
surgeons to assist or guide therapy for heart rhythm disturbances (arrhythmias). They are trained to perform interventional and surgical procedures to treat cardiac arrhythmia.
The training required to become an electrophysiologist is long
and requires 7 to 8 years after medical school (within the U.S.). Three
years of internal medicine residency, three years of Clinical Cardiology
fellowship, and one to two (in most instances) years of clinical
cardiac electrophysiology.
Cardiogeriatrics
Cardiogeriatrics,
or geriatric cardiology, is the branch of cardiology and geriatric
medicine that deals with the cardiovascular disorders in elderly people.
Echocardiography uses standard two-dimensional, three-dimensional, and Doppler ultrasound to create images of the heart.
Echocardiography has become routinely used in the diagnosis,
management, and follow-up of patients with any suspected or known heart
diseases. It is one of the most widely used diagnostic tests in
cardiology. It can provide a wealth of helpful information, including
the size and shape of the heart (internal chamber size quantification),
pumping capacity, and the location and extent of any tissue damage. An
echocardiogram can also give physicians other estimates of heart
function, such as a calculation of the cardiac output, ejection fraction, and diastolic function (how well the heart relaxes).
Echocardiography can help detect cardiomyopathies, such as hypertrophic cardiomyopathy, dilated cardiomyopathy,
and many others. The use of stress echocardiography may also help
determine whether any chest pain or associated symptoms are related to
heart disease. The biggest advantage to echocardiography is that it is
not invasive (does not involve breaking the skin or entering body
cavities) and has no known risks or side effects.
Interventional cardiology
Interventional cardiology is a branch of cardiology that deals specifically with the catheter based treatment of structural heart diseases.
A large number of procedures can be performed on the heart by
catheterization. This most commonly involves the insertion of a sheath
into the femoral artery (but, in practice, any large peripheral artery or vein) and cannulating the heart under X-ray visualization (most commonly Fluoroscopy).
The main advantages of using the interventional cardiology or
radiology approach are the avoidance of the scars and pain, and long
post-operative recovery. Additionally, interventional cardiology
procedure of primary angioplasty is now the gold standard of care for an acute Myocardial infarction. This procedure can also be done proactively, when areas of the vascular system become occluded from Atherosclerosis.
The Cardiologist will thread this sheath through the vascular system to
access the heart. This sheath has a balloon and a tiny wire mesh tube
wrapped around it, and if the cardiologist finds a blockage or Stenosis,
they can inflate the balloon at the occlusion site in the vascular
system to flatten or compress the plaque against the vascular wall. Once
that is complete a Stent is placed as a type of scaffold to hold the vasculature open permanently.
In recent times, the focus is gradually shifting to Preventive cardiology due to increased Cardiovascular Disease
burden at an early age. As per WHO, 37% of all premature deaths are due
to cardiovascular diseases and out of this, 82% are in low and middle
income countries.
Clinical cardiology is the sub specialty of Cardiology which looks
after preventive cardiology and cardiac rehabilitation. Preventive
cardiology also deals with routine preventive checkup though non
invasive tests specifically Electrocardiography, Stress Tests, Lipid Profile
and General Physical examination to detect any cardiovascular diseases
at an early age while cardiac rehabilitation is the upcoming branch of
cardiology which helps a person regain his overall strength and live a
normal life after a cardiovascular event. A subspecialty of preventive
cardiology is sports cardiology.
Pediatric cardiology
Helen B. Taussig is known as the founder of pediatric cardiology. She became famous through her work with Tetralogy of Fallot, a congenital heart defect in which oxygenated and deoxygenated blood enters the circulatory system resulting from a ventricular septal defect (VSD) right beneath the aorta. This condition causes newborns to have a bluish-tint, cyanosis, and have a deficiency of oxygen to their tissues, hypoxemia. She worked with Alfred Blalock and Vivien Thomas at the Johns Hopkins Hospital
where they experimented with dogs to look at how they would attempt to
surgically cure these "blue babies." They eventually figured out how to
do just that by the anastomosis of the systemic artery to the pulmonary artery and called this the Blalock-Taussig Shunt.
Tetralogy of Fallot is the most common congenital heart disease arising in 1–3 cases per 1,000 births. The cause of this defect is a ventricular septal defect (VSD) and an overriding aorta. These two defects combined causes deoxygenated blood to bypass the lungs and going right back into the circulatory system. The modified Blalock-Taussig shunt
is usually used to fix the circulation. This procedure is done by
placing a graft between the subclavian artery and the ipsilateral
pulmonary artery to restore the correct blood flow.
Pulmonary atresia
Pulmonary atresia
happens in 7–8 per 100,000 births and is characterized by the aorta
branching out of the right ventricle. This causes the deoxygenated blood
to bypass the lungs and enter the circulatory system. Surgeries can fix
this by redirecting the aorta and fixing the right ventricle and
pulmonary artery connection.
There are two types of pulmonary atresia, classified by whether or not the baby also has a ventricular septal defect.
Pulmonary atresia with an intact ventricular septum: This type of pulmonary atresia is associated with complete and intact septum between the ventricles.
Pulmonary atresia with a ventricular septal defect: This type of
pulmonary atresia happens when a ventricular septal defect allows blood
to flow into and out of the right ventricle.
Double outlet right ventricle
Double outlet right ventricle
(DORV) is when both great arteries, the pulmonary artery and the aorta,
are connected to the right ventricle. There is usually a VSD in
different particular places depending on the variations of DORV,
typically 50% are subaortic and 30%. The surgeries that can be done to
fix this defect can vary due to the different physiology and blood flow
in the defected heart. One way it can be cured is by a VSD closure and
placing conduits to restart the blood flow between the left ventricle
and the aorta and between the right ventricle and the pulmonary artery.
Another way is systemic-to-pulmonary artery shunt in cases associated
with pulmonary stenosis. Also, a balloon atrial septostomy can be done to fix DORV with the Taussig-Bing anomaly.
Transposition of great arteries
Dextro-transposition of the Great Arteries
There are two different types of transposition of the great arteries, Dextro-transposition of the great arteries and Levo-transposition of the great arteries,
depending on where the chambers and vessels connect.
Dextro-transposition happens in about 1 in 4,000 newborns and is when
the right ventricle pumps blood into the aorta and deoxygenated blood
enters the bloodstream. The temporary procedure is to create an atrial septal defect
(ASD). A permanent fix is more complicated and involves redirecting the
pulmonary return to the right atrium and the systemic return to the
left atrium, which is known as the Senning procedure. The Rastelli procedure
can also be done by rerouting the left ventricular outflow, dividing
the pulmonary trunk, and placing a conduit in between the right
ventricle and pulmonary trunk. Levo-transposition happens in about 1 in
13,000 newborns and is characterized by the left ventricle pumping blood
into the lungs and the right ventricle pumping the blood into the
aorta. This may not produce problems at the beginning, but will
eventually due to the different pressures each ventricle uses to pump
blood. Switching the left ventricle to be the systemic ventricle and the
right ventricle to pump blood into the pulmonary artery can repair
levo-transposition.
Persistent truncus arteriosus
Persistent truncus arteriosus is when the truncus arteriosus
fails to split into the aorta and pulmonary trunk. This occurs in about
1 in 11,000 live births and allows both oxygenated and deoxygenated
blood into the body. The repair consists of a VSD closure and the
Rastelli procedure.
Ebstein anomaly
Ebstein's anomaly
is characterized by a right atrium that is significantly enlarged and a
heart that is shaped like a box. This is very rare and happens in less
than 1% of congenital heart disease cases. The surgical repair varies
depending on the severity of the disease.
Pediatric cardiology is a sub-specialty of pediatrics. To become a pediatric cardiologist in the United States, one must complete a three-year residency in pediatrics, followed by a three-year fellowship in pediatric cardiology. Per doximity, pediatric cardiologists make an average of $303,917 in the United States.
The heart
Blood flow through the valves
As the center focus of cardiology, the heart has numerous anatomical features (e.g., atria, ventricles, heart valves) and numerous physiological features (e.g., systole, heart sounds, afterload) that have been encyclopedically documented for many centuries.
The primary responsibility of the heart is to pump blood throughout the body.
It pumps blood from the body — called the systemic circulation — through the lungs — called the pulmonary circulation — and then back out to the body.
This means that the heart is connected to and affects the entirety of the body. Simplified, the heart is a circuit of the Circulation.
While plenty is known about the healthy heart, the bulk of study in
cardiology is in disorders of the heart and restoration, and where
possible, of function.
The heart is a muscle that squeezes blood and functions like a
pump.
Each part of the heart is susceptible to failure or dysfunction and the
heart can be divided into the mechanical and the electrical parts.
The mechanical part of the heart is centered on the fluidic movement of blood and the functionality of the heart as a pump.
The mechanical part is ultimately the purpose of the heart and many of
the disorders of the heart disrupt the ability to move blood.
Failure to move sufficient blood can result in failure in other organs
and may result in death if severe.
Heart failure
is one condition in which the mechanical properties of the heart have
failed or are failing, which means insufficient blood is being
circulated.
As the left and right coronary arteries run on the surface of the
heart, they can be called epicardial coronary arteries. These arteries,
when healthy, are capable of autoregulation to maintain coronary blood
flow at levels appropriate to the needs of the heart muscle. These relatively narrow vessels are commonly affected by atherosclerosis and can become blocked, causing angina or a heart attack. The coronary arteries that run deep within the myocardium are referred to as subendocardial.
The coronary arteries are classified as "end circulation", since
they represent the only source of blood supply to the myocardium; there
is very little redundant blood supply, which is why blockage of these
vessels can be so critical.
Like all medical examinations, the cardiac examination follows the standard structure of inspection, palpation and auscultation.
Heart disorders
Cardiology is concerned with the normal functionality of the heart and the deviation from a healthy heart.
Many disorders involve the heart itself but some are outside of the heart and in the vascular system.
Collectively, the two together are termed the cardiovascular system and diseases of one part tend to affect the other.
Lifestyle factors can increase the risk of hypertension. These include excess salt in the diet, excess body weight, smoking, and alcohol. Hypertension can also be caused by other diseases, or as a side-effect of drugs.
Blood pressure is expressed by two measurements, the systolic and diastolic pressures, which are the maximum and minimum pressures, respectively. Normal blood pressure at rest is within the range of 100–140 millimeters mercury (mmHg) systolic and 60–90 mmHg diastolic. High blood pressure is present if the resting blood pressure is persistently at or above 140/90 mmHg for most adults. Different numbers apply to children. Ambulatory blood pressure monitoring over a 24-hour period appears more accurate than office best blood pressure measurement.
Lifestyle changes and medications can lower blood pressure and decrease the risk of health complications. Lifestyle changes include weight loss, decreased salt intake, physical exercise, and a healthy diet. If lifestyle changes are not sufficient then blood pressure medications are used. Up to three medications can control blood pressure in 90% of people.
The treatment of moderate to severe high arterial blood pressure
(defined as >160/100 mmHg) with medications is associated with an
improved life expectancy and reduced morbidity. The effect of treatment of blood pressure between 140/90 mmHg and 160/100 mmHg is less clear, with some reviews finding benefit and others finding a lack of evidence for benefit. High blood pressure affects between 16 and 37% of the population globally. In 2010 hypertension was believed to have been a factor in 18% (9.4 million) deaths.
Essential vs Secondary hypertension
Essential hypertension is the form of hypertension
that by definition has no identifiable cause. It is the most common
type of hypertension, affecting 95% of hypertensive patients, it tends to be familial and is likely to be the consequence of an interaction between environmental and genetic factors. Prevalence of essential hypertension increases with age, and individuals with relatively high blood pressure at younger ages are at increased risk for the subsequent development of hypertension.
Hypertension can increase the risk of cerebral, cardiac, and renal events.
Cardiac arrhythmia, also known as "cardiac dysrhythmia" or "irregular heartbeat", is a group of conditions in which the heartbeat is irregular, too fast, or too slow. A heart rate that is too fast – above 100 beats per minute in adults – is called tachycardia and a heart rate that is too slow – below 60 beats per minute – is called bradycardia. Many types of arrhythmia have no symptoms. When symptoms are present these may include palpitations or feeling a pause between heartbeats. More seriously there may be lightheadedness, passing out, shortness of breath, or chest pain. While most types of arrhythmia are not serious, some predispose a person to complications such as stroke or heart failure. Others may result in cardiac arrest.
Most arrhythmias can be effectively treated. Treatments may include medications, medical procedures such as a pacemaker, and surgery. Medications for a fast heart rate may include beta blockers or agents that attempt to restore a normal heart rhythm such as procainamide.
This later group may have more significant side effects especially if
taken for a long period of time. Pacemakers are often used for slow
heart rates. Those with an irregular heartbeat are often treated with blood thinners
to reduce the risk of complications. Those who have severe symptoms
from an arrhythmia may receive urgent treatment with a jolt of
electricity in the form of cardioversion or defibrillation.
Arrhythmia affects millions of people. In Europe and North America, as of 2014, atrial fibrillation affects about 2% to 3% of the population. Atrial fibrillation and atrial flutter resulted in 112,000 deaths in 2013, up from 29,000 in 1990. Sudden cardiac death is the cause of about half of deaths due to cardiovascular disease or about 15% of all deaths globally. About 80% of sudden cardiac death is the result of ventricular arrhythmias. Arrhythmias may occur at any age but are more common among older people.
Prevention is by eating a healthy diet, regular exercise, maintaining a healthy weight and not smoking. Sometimes medication for diabetes, high cholesterol, or high blood pressure are also used. There is limited evidence for screening people who are at low risk and do not have symptoms. Treatment involves the same measures as prevention. Additional medications such as antiplatelets including aspirin, beta blockers, or nitroglycerin may be recommended. Procedures such as percutaneous coronary intervention (PCI) or coronary artery bypass surgery (CABG) may be used in severe disease. In those with stable CAD it is unclear if PCI or CABG in addition to the other treatments improve life expectancy or decreases heart attack risk.
In 2013 CAD was the most common cause of death globally, resulting in 8.14 million deaths (16.8%) up from 5.74 million deaths (12%) in 1990. The risk of death from CAD for a given age has decreased between 1980 and 2010 especially in developed countries. The number of cases of CAD for a given age has also decreased between 1990 and 2010. In the United States in 2010 about 20% of those over 65 had CAD, while it was present in 7% of those 45 to 64, and 1.3% of those 18 to 45. Rates are higher among men than women of a given age.
The most common cause of cardiac arrest is coronary artery disease. Less common causes include major blood loss, lack of oxygen, very low potassium, heart failure, and intense physical exercise. A number of inherited disorders may also increase the risk including long QT syndrome. The initial heart rhythm is most often ventricular fibrillation. The diagnosis is confirmed by finding no pulse. While a cardiac arrest may be caused by heart attack or heart failure these are not the same.
In the United States, cardiac arrest outside of hospital occurs in about 13 per 10,000 people per year (326,000 cases). In hospital cardiac arrest occurs in an additional 209,000 Cardiac arrest becomes more common with age. It affects males more often than females. The percentage of people who survive with treatment is about 8%. Many who survive have significant disability. Many U.S. television shows, however, have portrayed unrealistically high survival rates of 67%.
Congenital heart defects
A congenital heart defect, also known as a "congenital heart anomaly" or "congenital heart disease", is a problem in the structure of the heart that is present at birth. Signs and symptoms depend on the specific type of problem. Symptoms can vary from none to life-threatening. When present they may include rapid breathing, bluish skin, poor weight gain, and feeling tired. It does not cause chest pain. Most congenital heart problems do not occur with other diseases. Complications that can result from heart defects include heart failure.
The cause of a congenital heart defect is often unknown. Certain cases may be due to infections during pregnancy such as rubella, use of certain medications or drugs such as alcohol or tobacco, parents being closely related, or poor nutritional status or obesity in the mother. Having a parent with a congenital heart defect is also a risk factor. A number of genetic conditions are associated with heart defects including Down syndrome, Turner syndrome, and Marfan syndrome. Congenital heart defects are divided into two main groups: cyanotic heart defects and non-cyanotic heart defects, depending on whether the child has the potential to turn bluish in color. The problems may involve the interior walls of the heart, the heart valves, or the large blood vessels that lead to and from the heart.
Congenital heart defects are partly preventable through rubella vaccination, the adding of iodine to salt, and the adding of folic acid to certain food products. Some defects do not need treatment. Other may be effectively treated with catheter based procedures or heart surgery. Occasionally a number of operations may be needed. Occasionally heart transplantation is required. With appropriate treatment outcomes, even with complex problems, are generally good.
Heart defects are the most common birth defect. In 2013 they were present in 34.3 million people globally. They affect between 4 and 75 per 1,000 live births depending upon how they are diagnosed. About 6 to 19 per 1,000 cause a moderate to severe degree of problems. Congenital heart defects are the leading cause of birth defect-related deaths. In 2013 they resulted in 323,000 deaths down from 366,000 deaths in 1990.